The present invention relates in general to decoding frequency-shift keying (FSK) signals, and more specifically to an FSK decoding scheme having a fast response time by avoiding integration in the decoder.
Frequency-shift keying is a well-known method for representing binary data using a first frequency to represent a binary zero and a second frequency to represent a binary one. When the binary digit to be represented changes, the frequency of the FSK signal shifts from one frequency to the other. Although the frequency shifts abruptly, FSK systems typically employ a continuous phase, i.e., there is no phase discontinuity when the frequency shifts.
Frequency-shift keying is a type of frequency modulation and can be used to transmit digital information by radio waves. FSK modulation is also used in transmitting binary information over phone lines using modems.
Demodulation of FSK signals typically employs FM detection techniques which include filtering with long time constants or other integration of the FSK signal. For example, the FSK signal to be demodulated is applied to the inputs of a pair of bandpass filters having respective center frequencies corresponding to the two frequencies of the FSK signal. The filter outputs are coupled to the inputs of a differential amplifier which recreates the digital signal. However, the integration time constants of the filters introduce a time delay slowing down the response time of the FSK demodulator.
Other demodulation techniques rely on the use of quadrature mixing signals to generate quadrature and in-phase FSK components for detection, resulting in high part count and high cost.